Such a converter-fed magnetic bearing is known from DE 10 2007 028 229 B3. A converter-fed magnetic bearing of this type is shown in more detail in FIG. 1. This FIG. 1 refers to a converter-fed magnetic bearing with 2, a converter with 4, an inverter with 6, an upper magnetic armature with 8, a lower magnetic armature with 10, a coil of the upper magnetic armature 8 with 12, a coil of the lower magnetic armature 10 with 14, a feed unit of the converter 4 with 16 and a feeding network with 18. In this representation the coil 12 of the upper magnetic armature 8 is connected with a first connection 20 to an output W of the inverter 6, wherein a first connection 22 of the coil 14 of the lower magnetic armature 10 is connected to an output V of the inverter 6. The second connections 24 and 26 of the two coils 12 and 14 are connected to one another and to a third output U of the inverter 6. The converter 4, in addition to the inverter 6, also still comprises a feed unit 16, which is connected on the AC voltage side to a feeding network 18. The feed unit 16 is connected on the DC voltage side to DC voltage side connections DC+ and DC− of the inverter 6. The inverter 6 is available on the market as a motor module for a conventional converter device.
An inverter device of this type is normally used to activate a three-phase motor. To ensure that a three-phase motor, in particular a synchronous motor, can be regulated in accordance with standards in respect of rotational speed or torque, this motor module comprises a field-oriented regulator. In other words, this regulator has two regulating channels, namely one channel for a so-called field-oriented current component, which is generally referred to as the d-component of a rotating current vector, and one channel for a so-called torque-forming current component, which is generally referred to as the q-component of a rotating current vector. According to the cited national patent, these two current components, which are to be generated independently of one another, are used for a differential activation of a magnetic bearing. The d-component is in this case assigned the function of a constant current for bias current, wherein the role of the control current of the magnetic bearing, with which the force effect is controlled, is incumbent on the q-component.
A mode of operation of the converter-fed magnetic bearing 2 according to FIG. 1 is known from this DE 10 2007 028 229 B3, which is explained in more detail with the aid of FIGS. 2 to 4.
According to this mode of operation, a transformation angle (rotor position angle of a synchronous motor) of the field-oriented regulation of the inverter 6 is frozen such that the state arises for the inverter 6 as if the magnetic axis of a driven motor permanently and invariably points in a specific direction of the stator, for instance in the direction of the phase at output U. A stator-oriented right-angled α/β system is transformed into a rotor-oriented d/q system by means of this transformation angle (rotor position angle), which rotates with a rotor circuit frequency. This rotor position angle is also required again for the back transformation. This transformation angle (rotor position angle) changes during a rotation of the rotor, as a result of which the variable rotor position angle is also referred to as a rotary field angle.
With the mode of operation of the inverter 6 as a feed unit for a magnetic bearing, this transformation angle is frozen such that the flux-forming axis permanently and invariably points in the direction of the phase of the output U of the three-phase inverter. In other words, the transformation angle is constantly predetermined at zero degrees.
In this determination of the transformation angle, the d-current component id then flows, in equal parts, into the phase conductor at the outputs V and W. The phase current iu in the phase at the output U of the three-phase inverter (FIG. 2) is thus twice as large as the current id. The arrow directions in FIG. 2 indicate the direction in which a current is to be evaluated as positive. Conversely the q-current component iq flows out of the phase at output W directly into the phase at output V of the three-phase inverter, wherein the phase at the output U of this three-phase inverter is not affected (FIG. 3). In FIG. 4, the addition of the d- and q-current components id and iq is shown at full scale of the converter-fed magnetic bearing.
The effective current is crucial to the thermal load of the inverter 6. The effective current can be understood to be the direct current which an equivalent heat output would generate in an imaginary 1 ohm resistor. Since the inverter 6 outputs three phase currents, an associated replacement direct current acts in three resistors with 1 Ohm in each instance. During operation with id=10 A from the output U of the inverter 6, a current iu=−20 A and into the outputs V and W of the inverter 6 a current iv=10 A and iw=10 A respectively. An effective current of 14.14 Aeff is produced.
With magnetic bearings, the full scale level is generally defined such that at full scale the current linkage (number of ampere turns) for instance in the coil 12 of the upper magnetic armature of the magnetic bearing 2 just about disappears, while it doubles in the coil 14 of the lower magnetic armature of this magnetic bearing compared with a basic current linkage (FIG. 4). At full scale level of the magnetic bearing, the effective current with the already specified current values has a value of 16.33 Aeff. In other words, the effective value only changes to a minimal degree between the states bias current and full scale (FIG. 11). According to this known mode of operation, the bias current (d-current component) according to the known operating method (DE 10 2007 028 B3) is injected into the phase at output U of the inverter 6.
According to the effective current load of the inverter 6, the velocity of the change in current is an important feature of a magnetic bearing activation. The quicker the magnetic bearing can change the control current and thus the force, the better it can respond to a dynamic force requirement, such as for instance imbalance or a sudden external load affecting it.
The velocity of the change in current is proportional to the voltage difference which can be applied at the two connections 22 and 20 of the two coils 14 and 12 of the converter-fed magnetic bearing 2. As a maximum voltage difference, the inverter 6 can apply the intermediate circuit voltage present at its DC voltage side connections DC+ and DC− in any manner to its outputs U, V and W. The intermediate circuit voltage UZK is offered by an intermediate circuit capacitor 28 of the converter 4, which is therefore also referred to as voltage intermediate circuit converter. An intermediate circuit voltage UZK is a rectified network voltage which is generated by means of the feed unit 16. With the magnetic bearing controller according to DE 10 2007 028 229 B3, the control current (iq-current component of the inverter) flows between the outputs V and W of the three-phase inverter 6. The quickest change in this control current is achieved if the inverter 6 is activated such that its output W is connected to the reference potential of the voltage intermediate circuit and its output V is connected to a positive potential of the voltage intermediate circuit of the converter 4. According to this controller, the intermediate circuit voltage UZK is applied to the outputs V and W of the three-phase inverter 6, and is thus applied to the series circuit of the two coils 12 and 14 of the converter-fed magnetic bearing 2 (FIG. 5). If it is assumed that the two coils 12 and 14 of this magnetic bearing 2 each have an inductance L, the rate of current rise is produced in accordance with the following equation:
            Δ      ⁢                          ⁢      I              Δ      ⁢                          ⁢      t        =            U      ZK              2      ·      L      
The two inductances L of the two electrically series-connected coils 12 and 14 of the magnetic bearing are opposed by the change in current which is driven by the voltage UZK.